What exactly are you worried about being copied? The entire site (files and database), content, images, other??

As a note, for what you’re asking, someone would jsut copy the files, and that doesn’t have anything to do with any of the code in WordPresss, your plugin, or anything else. You just need a secure account on your server, as nothing else will work.

To be honest, you can set up something like this. But… Your plugins code is open-source, so if someone really wants it, they’ll just edit your plugins code to remove the part about licence keys.

The best thing that you can do is sell plugins the wya that Woo, and alot of other places, do it. Don’t sell the plugin as such, but sell access to updates and support. That way your plugin can be used, but you’ll be able to manage updates and only give support to paid subscribers.

There’s things that you can add in, like blocking execution if the domain name doesn’t match, but as I said, anyone with a small bit of PHP experience can edit out those bits of code anyway.

So, no. You cannot stop someone from copying your plugins code in your case. You will have to trust your client, or set up a bullet-proof contract allowing them a single use of the plugin on a single site.